1. Field of the Invention
The present invention relates to a ladder filter and more particularly, to a ladder filter constructed for use in processing signals of a KHz band in an intermediate frequency amplifying stage of an AM receiver and similar devices.
2. Description of Related Art
A related ladder filter 20 is shown in FIGS. 8 and 9 and disclosed in Japanese Patent Laid-Open No. Hei. 6-334473. The ladder filter 20 includes a case 1 having two internal spaces 2 and 3 separated by a partition 4. Each of the internal spaces 2 and 3 has an opening at an upper part thereof. Piezoelectric resonators 5-10 and terminals 11-17 are arranged so as to overlap each other in the internal spaces 2 and 3, respectively. That is, the terminal 11, the piezoelectric resonator 5, the terminal 12, the piezoelectric resonator 6, the terminal 13, the piezoelectric resonator 7 and the terminal 14 are laminated within the internal space 2 in this order. The terminal 15, the piezoelectric resonator 8, the terminal 11, the piezoelectric resonator 9, the terminal 16, the terminal 12, the piezoelectric resonator 10 and the terminal 17 are laminated within the internal space 3 in this order.
The terminal 11 upon which the piezoelectric resonator 5 is disposed is also located between the piezoelectric resonators 8 and 9. The terminal 12 disposed between the piezoelectric resonators 5 and 6 is extended between the piezoelectric resonator 10 and the terminal 16. The terminal 12 is electrically insulated from the terminal 16 which is in contact with the piezoelectric resonator 9.
The ladder filter 20 has a circuit structure as shown in FIG. 9, wherein the piezoelectric resonators 8, 5 and 6 are series piezoelectric resonators of the ladder filter circuit and the piezoelectric resonators 9, 10 and 7 are parallel piezoelectric resonators of the ladder filter circuit. The terminal 15 functions as an input terminal, the terminals 14, 16 and 17 function as ground terminals and the terminal 13 functions as an output terminal, respectively.
The two terminals 12 and 16 are disposed between the piezoelectric resonators 9 and 10 in the prior art ladder filter 20. The terminal 12 functions as a relay terminal for electrically connecting the series piezoelectric resonator 5 with the series piezoelectric resonator 6 and the other terminal 16 functions as the ground terminal, such that the terminal 12 must be electrically insulated from the terminal 16 within the case 1.
The terminals 12 and 16 are insulated by interposing an insulator sheet between the terminals 12 and 16 or disposing a coating insulator on either one or both of the terminals 12, 16. Accordingly, the structure of the ladder filter 20 is complicated which increases the difficulty, time and cost involved in manufacturing and assembling the filter 20. In addition, the above-described construction and configuration of the components of the filter 20 has made it very difficult to miniaturize the filter and to assure high reliability.
Furthermore, because the five terminals 13-17 extend out of the case 1 as seen in FIG. 9, the cost for molding the terminals is very high and the rate of failure in reliably mounting the filter 20 and terminals 13-17 on a printed board increases as the number of terminals increases.